The fabrication of precisely engineered polymeric nanostructures with control over their morphology and composition is vital for wide ranging applications from nanomedicine to material science (1). Synthesis of polymeric nanostructures is often achieved by self-assembly (2) or emulsion based “bottom-up” strategies, with limited control over size and shape (3). A wide range of templates like anodic aluminum oxide (4), mesocellular silica foam (5), liposomes (6), perfluoropolyether molds (7) etc. have been used for the synthesis of morphologically distinct polymers with varying degrees of success. In spite of the recent advances in organic synthesis and polymerization methods (8), there exists a strong need to develop new, simpler and direct routes to polymeric nanostructures with well-defined size, morphology and surface composition.
Among various polymeric architectures, hollow nanocapsules in particular have attracted enormous attention in recent years as drug delivery vehicles, nanoreactors, and in catalysis (9). While numerous approaches are available for the fabrication of nanocapsules and related nanocontainers (10) functionalization strategies are still in their infancy (10i)(10j). A variety of self-assembled nanocapsules mimicking biological systems (9)(10e) such as liposomes, polymersomes, and layer-by-layer capsules (10a) have been investigated. When compared to the non-covalently assembled structures, covalently linked polymeric nanocapsules offer crucial mechanical stability mandatory for a variety of applications (9). In the more popular template based approach a polymeric shell is grown around the template by a variety of means, following which the sacrificial template is selectively removed, resulting in the formation of nanocapsules (10). A number of templates including gold (10g) and silica nanoparticles (10a)(10f) self-assembled amphiphilic block copolymers (10a)(10c) and dendrimers (10k) have been employed for the synthesis of nanocapsules.
Over the years several surfactant stabilized emulsions have also been used for the synthesis of nanocapsules (10e). The techniques currently employed for the fabrication of nanocapsules require one of the following: self-assembly, sacrificial templates or emulsifiers (10e). These strategies have certain intrinsic limitations such as tedious procedures for the removal of sacrificial template or surfactants, lack of nanocapsule robustness, low efficiency etc. (10f)(10l).
Recently, Kim and coworkers have reported a direct synthesis of nanocapsules by employing thiol-ene photopolymerization of allyloxy cucurbituril with dithiol linkers (12). Resorcinarenes (13) are a well-established building block in supramolecular chemistry, with a rigid head and flexible tails, and are easily modified at either end. We envisaged that multiple thiol and ene functionalities could be introduced in a resorcinarene molecule, and most importantly these functionalities could be selectively placed in distinct locations, i.e., in the macrocyclic head or alkyl tail groups with widely differing rigidity, potentially resulting in novel nanostructures upon polymerization. Wei and coworkers have recently reported the use of RTATT, as a crosslinkable surfactant around gold nanoparticles using olefin-metathesis (14).
Surprisingly, it has been discovered that template-free polymerization of resorcinarenes is possible and that these resorcinarenes can be used to form nanocapsules and other polymeric architectures like lattices, fibrous network structures, and nanoparticles. The synthesis of the resorcinarene polymers are achieved by thiol-ene photopolymerization (11), a versatile polymerization tool with widespread applications and several attractive features like initiator-free initiation, functional group tolerance, and oxygen inhibition resistance.